1. Field of the Invention
The present invention relates to a surface acoustic wave device comprising inter-digital-transducers (hereinafter refers to as IDT) formed on a piezoelectric substrate.
2. Description of the Related Art
So far, surface acoustic wave devices, due to small size, high performance and high reliability thereof, have been used in various fields. For instance, in the field of radio communication such as portable telephones or the like, the surface acoustic wave devices are used as band-pass filters for transmission, band-pass filters for reception or the like.
In the field of radio communication as such, there is a tendency toward allowing coexistence of two or more systems of different frequency bands. For instance, in North America and so on there exist two systems of PCS of 1.9 GHz band and AMPS of 800 MHz band, in Europe and so on two systems of DCS of 1.8 GHz band and GSM of 900 MHz band, and in Japan a plurality of systems of PHS of 1.9 GHz band, PDC1.5G of 1.5 GHz band and PDC800 of 800 MHz band.
Accordingly, there is a recent tendency demanding dual-type radio communication units that can materialize communication in two or more systems of different frequency bands with one unit. In such dual-type radio communication units, various kinds of components can be used in common. Accordingly, compared with the case where one radio communication unit is prepared for each of different systems, from a viewpoint of total cost and size, there is a considerable merit.
For a surface acoustic wave device capable of coping with such a dual-type radio communication unit, one in which a plurality of surface acoustic wave chips operating in mutually different frequency bands is accommodated in one package can be used. As a result of this, the surface acoustic wave device can be made small and less expensive.
In a surface acoustic wave device, ordinarily surrounding an area thereon a surface acoustic wave chip is mounted, bonding pads are disposed for supplying input signals and ground potential to IDTs from external portions, or extracting output signals. These bonding pads and terminals of the IDTs are connected with bonding wires.
In the case of accommodating a plurality of surface acoustic wave chips as such in one package to prepare so-called multi-chip package, terminals of the IDTs are connected to a bonding pad on one side of each chip. Accordingly, depending on locations of the terminals of the IDTs, the lengths of the bonding-wires can be different.
As a result of this, in the case of arranging a plurality of IDTs along a main propagation direction of surface acoustic waves such as in particular in a longitudinal mode-coupled resonant filter, an influence of a capacitance component parasitic on the bonding wire and an inductance component is different for each IDT. Thereby, it is likely to cause difficulty in adjusting frequency characteristics. Further, when the bonding-wire goes over the IDT, electromagnetic coupling tends to occur between the bonding-wire and the IDT.
The present invention is carried out for improving a surface acoustic wave device and has the following objects.
(1) To provide a surface acoustic wave device easy in packaging in a multi-chip package.
(2) To provide a surface acoustic wave device having IDTs(inter-digital transducer) of even characteristics through homogenizing parasite capacitance components and inductance components for all IDTs.
(3) To prevent signal and noise from mingling due to electromagnetic coupling between bonding-wires and IDTs.
(4) To provide a surface acoustic wave device of small size.
A surface acoustic wave device of the present invention comprises a package and a plurality of surface acoustic wave chips. Here, the package comprises a base on one main surface thereof a conductor is disposed. The plurality of surface acoustic wave chips comprises a plurality of IDTs formed on one main surface of a piezoelectric substrate, signal terminals disposed on both opposite sides on the piezoelectric substrate, and a ground terminal disposed on the piezoelectric substrate. Here, the plurality of surface acoustic wave chips, at the signal terminals and the ground terminal, are bonded to the conductor in a face down way.
In a surface acoustic wave device of the present invention, power supply to and extraction of output from each IDT terminal of multi-chips are implemented through a conductor on a base. Accordingly, the difference of an influence of the parasite capacitance components and the inductance components due to locations of the IDT terminals and an influence of electromagnetic coupling can be alleviated. As a result of this, multi-chip packages can be manufactured with ease.
Further, on the base signal conductors corresponding to a plurality of signal terminals of a chip may be formed and on an area between these adjacent signal conductors part of ground conductor may be formed. Thereby, since electromagnetic coupling between adjacent signal conductors can be shielded by the ground conductor that is intervened therebetween, an influence of the electromagnetic coupling between the signal conductor can be alleviated.
In addition, all of the IDTs may be disposed facing the ground conductor. Thereby, the IDTs can be prevented from being mingled with electrical signal and noise.
At least one of surface acoustic wave chips may be constituted of a plurality of surface acoustic wave filters connected in a cascade connection.
At least one of surface acoustic wave chips can be constituted of a longitudinal mode-coupled surface acoustic wave element.
A longitudinal mode-coupled surface acoustic wave resonant filter, one example thereof, is normally constituted by arranging a plurality of IDTs and reflectors disposed at both ends sandwiching these IDTs in rows along a direction of propagation of surface acoustic waves on a piezoelectric substrate. As a result of this, in addition to a fundamental wave of surface acoustic waves, a higher mode thereof can be extracted. As a result of this, a filter of a broader band can be obtained.
When mounting this longitudinal mode-coupled surface acoustic wave resonator as a multi-chip, according to the present invention, fluctuation of value of parasite capacitance and value of inductance of each IDT can be alleviated. As a result of this, surface acoustic waves of higher mode can be efficiently extracted.
In a surface acoustic wave device of the present invention, an external selection circuit may be disposed to supply input signal selectively to any one of a plurality of surface acoustic wave chips, resulting in constituting a filter that can be switched.
Propagation characteristics of a piezoelectric substrate of at least one chip of surface acoustic wave chips may be different from that of other surface acoustic wave chips.
Without restricting to the propagation characteristics of the substrate, thickness of electrode fingers and constituent material of the electrode fingers (including composition or layer structure), or duty ratio (width of an electrode finger/distance between centers of electrode fingers) of the electrode fingers can be made different from those of other surface acoustic wave chips.
An external dimension of at least one chip of the surface acoustic wave chips may be different from that of other chips. Thereby, the chips can be distinguished with ease. Here, by designing the piezoelectric substrates to the minimum dimension required by each filter, low cost and lightweight can be realized.
When, as the external dimension, thicknesses of the piezoelectric substrates are differentiated according to the kinds of the surface acoustic wave chips, the surface acoustic wave chips can be conveniently distinguished with ease during and after mounting of the chips. It is preferable for the difference of the thicknesses of the substrates to be 20 xcexcm or more for easy distinction.
In mounting the chips on a package, the chips can be mounted in order of thickness from one that is thin in its substrate thickness. Thereby, a mounting tool and a surface acoustic wave chip can be prevented from interfering with each other, resulting in easy mounting.
When a thickness of a piezoelectric substrate of at least one chip of surface acoustic wave chips is made different from that of other surface acoustic wave chips, according to bandwidth and rise transition characteristics required for the respective surface acoustic wave elements, the piezoelectric substrates of optimum thicknesses can be used.
Surface roughness of a rear surface of at least one chip of the surface acoustic wave chips can be made different from that of other chips. Thereby, the chips can be distinguished with ease.